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Aortic Dissection: True or False?

Malik, Zahra M. MD*; Lau, Christopher MD; Skubas, Nikolaos J. MD, DSc, FASE, FACC

doi: 10.1213/XAA.0000000000000748
Echo Didactics
Free
SDC

From the *New York-Presbyterian Weill Cornell Medical Center, New York, New York

Departments of Cardiothoracic Anesthesiology and Cardiothoracic Surgery, Weill Cornell Medicine, New York, New York

Department of Cardiothoracic Anesthesiology, Cleveland Clinic, Cleveland, Ohio.

Accepted for publication January 11, 2018.

Funding: None.

The authors declare no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Address correspondence to Zahra M. Malik, MD, Department of Anesthesiology, Weill Cornell Medicine 525 E 68th St, Box 124 New York, NY 10065. Address e-mail to zam9011@med.cornell.edu.

Figure 1.

Figure 1.

The patient provided written consent for this publication. A 64-year-old man with syncope is found to have a type A aortic dissection and presents to the operating room (Figure 1). Before aortic cannulation, the surgeon inquires whether the anterior lumen visualized on transesophageal echocardiography (TEE) is the true or the false lumen.

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DISCUSSION

Aortic dissection involves a disruption (tear) of the medial layer of the aortic wall that results in separation of the intima from the adventitia, allowing blood to track in a dissection plane within the media.1 This classically results in an aorta with 2 lumens separated by a dissection flap: a true lumen, which is in continuity with an undissected portion of the aorta and covered circumferentially of endothelium, and a false lumen, in which the walls are partially media and adventitia. The false lumen is perfused via the tear and fenestrations in the dissection flap. Ascending aortic dissection (DeBakey type I or II and Stanford type A) requires immediate surgical management. If left untreated, the mortality rate reaches approximately 50% at 48 hours.1 Absence of ascending aortic involvement (DeBakey type III or Stanford type B) may be managed medically or by endovascular stenting.

Risk factors associated with the development of aortic dissection include connective tissue disorders (Marfan and Ehlers-Danlos syndromes), inflammatory vascular diseases, existing bicuspid aortic valve or aortic aneurysm, and predisposing conditions such as hypertension, pregnancy, or pheochromocytoma, which ultimately strain and weaken the aortic wall.1 Trauma from deceleration and iatrogenic injury are causes of both type A (most commonly involving the right coronary ostium in coronary procedures) and type B aortic dissections.1,2 Aortic dissection can be identified on TEE, with a sensitivity and specificity of 98% and 95%, respectively, similar to computed tomography and magnetic resonance imaging.3 Comprehensive TEE evaluation in aortic dissection should include confirmation and extent of dissection, differentiation of the true from the false

lumen, identification of the entry tear, and assessment of complications (aortic regurgitation, coronary artery and major vessel involvement by dissection or dynamic obstruction, pleural and/or pericardial effusion).1,3 Three-dimensional (3D) TEE may delineate the spatial relationship between the dissection flap and surrounding structures not fully appreciated on 2D TEE.4

While there is a lack of consensus on the optimal arterial cannulation site for aortic repair, differentiation of the true from the false lumen in acute type A aortic dissection is crucial in guiding surgical management.5 Peripheral axillary and femoral cannulation is accomplished by direct visualization, and a guidewire placed in the distal femoral artery can be confirmed in the true lumen of the descending thoracic aorta on TEE. Direct central cannulation of the ascending aorta or aortic arch can be achieved by accessing the true lumen via a guidewire under TEE and epiaortic ultrasound guidance. This ensures that the true (and not the false) lumen is perfused during cardiopulmonary bypass, thus avoiding propagation of the dissection, aortic wall rupture, and ultimately malperfusion to vital organs.6 In our case, central cannulation of the ascending aorta was performed by placing a guidewire using epiaortic ultrasound on the surgical field with confirmation of the wire in the true lumen by TEE. Access of the true lumen is important during placement of intra-aortic balloon pump, endovascular stent grafting, and fenestration procedures.

Table.

Table.

While generalizations can be made differentiating the true and false lumens, echocardiographers should bear in mind that the anatomic and flow relationships described below are not absolute. Additionally, relative differences may not be obvious in every case. We review anatomic and flow characteristics of the true and false lumens using TEE, and common diagnostic pitfalls (Table).

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ANATOMIC CONSIDERATIONS

The dissection flap is seen as an echogenic, intraluminal, thin linear structure with variable motion, visualized on M-mode imaging with sufficient temporal resolution (Supplemental Digital Content 1, Supplemental Figure 1, http://links.lww.com/AACR/A162). It can be imaged in >1 echocardiographic imaging plane and should be distinguished from artifacts (discussed below).3 3D TEE demonstrates varying thickness along the dissection flap, distinguishing a relatively immobile dissection flap from artifact.3 When viewed in short axis, the dissection flap is most commonly concave toward the true lumen (Figure 1B) in acute aortic dissections, but may appear flat in chronic aortic dissections.7 The surface of the dissection flap facing the true lumen contains endothelium and anatomic findings associated with atherosclerosis, such as intimal thickening and calcification (Figure 1B; Supplemental Digital Content 2, Video 1, http://links.lww.com/AACR/A163).7

Luminal size, expansion, and thickness can also be used to differentiate the true from the false lumen. The true lumen is generally smaller than the false lumen.1,3,7,8 This is thought to be due to loss of transmural pressure across the dissection flap, which is severed from its connective tissue attachments to the media and adventitia. This results in elastic recoil of the dissection flap and expansion of the thinner, elastin-poor outer false lumen wall.8 The true lumen may expand in systole as the dissection flap is displaced toward the false lumen, while the dissection flap is displaced toward the true lumen in diastole, sometimes compressing it (Supplemental Digital Content 3, Video 2, http://links.lww.com/AACR/A164).1,3 This can be appreciated on simultaneous orthogonal plane imaging. M-mode is also useful in tracking rapid systolic displacement of the dissection flap during the cardiac cycle (Supplemental Digital Content 1, Supplemental Figure 1, http://links.lww.com/AACR/A162). The true lumen wall will be thicker than the aortic wall of the false lumen, because it comprises all 3 aortic wall layers, while the false lumen wall is thinner, because it is formed by adventitia and remnants of the media, best appreciated in the short-axis view (Figure 1B; Supplemental Digital Content 2, Video 1, http://links.lww.com/AACR/A163). These anatomic features can be appreciated on TEE imaging in several segments of the aorta.

Spontaneous echocardiographic contrast (SEC) and thrombosis suggestive of blood stasis are found in the false lumen (Supplemental Digital Content 1, Supplemental Figure 1, http://links.lww.com/AACR/A162; Supplemental Digital Content 3, Video 2, http://links.lww.com/AACR/A164).1,3 Detection of SEC requires appropriate gain settings (ie, excessive gain may produce white noise artifact in the true lumen, while SEC may be missed in the false lumen if gain settings are too low) and use of a high-frequency ultrasound transducer (ultrasound backscatter intensity is proportional to transducer frequency).9 In chronic aortic dissection, the false lumen may be completely thrombosed and only the true lumen may be visible.3

TEE findings mimic radiographic characteristics described for the false lumen in acute aortic dissection. The “beak sign” represents the acute angle formed between the dissection flap and false lumen wall seen on cross-sectional imaging (Figure 1B; Supplemental Digital Content 2, Video 1, http://links.lww.com/AACR/A163), sometimes associated with hematoma.7 Last, “cobwebs” are strands of incompletely sheared media in the false lumen.7

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FLOW CHARACTERISTICS

Under normal conditions, the aorta has predominantly anterograde flow with some diastolic retrograde flow in the ascending aorta and arch contributing to the coronary and cerebral circulations.10

Aortic blood flow can be evaluated using color-flow Doppler (CFD) and pulsed-wave Doppler (PWD). In the dissected aorta, PWD of the true lumen typically displays systolic antegrade velocity (Figure 2A).11,12 Rapid velocity creates a bright shade of red or blue on CFD, provided that the angle of incidence between the ultrasound beam and blood flow is not perpendicular.3 In comparison, the false lumen usually displays delayed and decreased (Figure 2B) or absent systolic antegrade velocity and in some cases diastolic velocity reversal.12 The peak systolic velocity is generally higher in the true than the false lumen (Figure 2) with a higher degree of turbulence in the false lumen.3,11

Figure 2.

Figure 2.

Entry tears are visualized on TEE as a disruption of the dissection flap continuity and can be visualized with CFD or intravenous contrast enhancement as a communication between the true lumen and false lumen (Supplemental Digital Content 4, Video 3, http://links.lww.com/AACR/A165).1,3 3D TEE may further delineate the size of the entry tear and its location relative to other structures in greater detail than 2D TEE, especially when the dissection flap spirals along the long axis of the aorta.3

TEE evaluation of the distal ascending aorta and aortic arch is limited by the “blind-spot” caused by the interposition of the air-filled trachea. In a novel approach, a fluid-filled balloon placed in the trachea and left main bronchus can improve visualization of the innominate and carotid arteries.13 Alternatively, epiaortic echocardiography using a high-resolution (>7 MHz) ultrasound transducer in the operating field can confirm the diagnosis of aortic dissection and guide aortic cannulation for definitive surgery when the ascending aorta and aortic arch cannot be visualized on TEE or when TEE probe insertion is contraindicated or cannot be performed.14 TEE is also limited when used to visualize a dissection extending into the distal abdominal aorta due to abdominal gas, variable aortic position, and relative posterior and leftward position of the descending thoracic aorta in relation to the esophagus.1,3,15

Intramural hematoma represents hemorrhage into the medial aortic layer. It is distinguished from aortic dissection by absence of a dissection flap, is more localized, with crescentic or concentric aortic wall thickening >5 mm. Additionally, there is absence of Doppler flow communication between the true and false lumens, but flow (with CFD) may be seen within the hematoma.1,3

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COMMON DIAGNOSTIC PITFALLS

Mirror artifacts produce a duplicate image distal to the real structure. Reflections from the anterior wall of the left atrium or right pulmonary artery may produce artifacts in the presence of a dilated ascending aorta.15 The anterior aortic wall can be duplicated when imaging the descending aorta and aortic arch in short- or long-axis views, respectively, producing a “double barrel aorta” that may be misinterpreted as an aortic dissection (Figure 3A).16,17 It is important to note that both CFD and PWD signals (Figure 3B, C) will be duplicated in the mirrored structure as well.17 Side-lobe artifacts appear as a radial “arc-like” artifact that traverses anatomic boundaries.16,17 Calcium at the sinotubular junction (mid-esophageal aortic valve long-axis view) and the anterior wall of the aortic arch (upper esophageal aortic arch long-axis view) can produce side-lobe artifact mimicking an ascending aortic dissection.16,17 Reverberation artifacts appear as a linear or “step ladder” artifact diminishing in intensity distal to the true structure.16,17 Calcification of the posterior aortic wall may produce this artifact that can be mistaken for an intimal flap in the descending aorta.17 Similarly, a pulmonary artery catheter may give rise to this artifact in the mid-esophageal aortic valve long-axis view.17

Figure 3.

Figure 3.

Alternate imaging planes should be used if an artifact is suspected as true structures are seen in multiple views and are well defined (Supplemental Digital Content 5, Supplemental Table 1, http://links.lww.com/AACR/A166). In comparison, artifacts cross through cardiac or vascular walls, display indistinct borders, and do not disturb surrounding CFD.17

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DISCLOSURES

Name: Zahra M. Malik, MD.

Contribution: This author helped design the study and write the manuscript.

Name: Christopher Lau, MD.

Contribution: This author helped design the study and write the manuscript.

Name: Nikolaos J. Skubas, MD, DSc, FASE, FACC.

Contribution: This author helped design the study and write the manuscript.

This manuscript was handled by: W. Scott Beattie, PhD, MD, FRCPC.

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REFERENCES

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